1. Field of the Invention
This invention relates to a new process for the production of polyureas by reacting blocked isocyanates with organic compounds containing at least two primary and/or secondary amino groups using guanidines of the type described in more detail below as catalysts for the polycondensation reaction.
2. Description of the Prior Art
The production of high molecular weight compounds containing urea groups using polyisocyanates is known, for example, from German Offenlegungsschrift No. 1,644,813 or U.S. Pat. No. 3,245,961. The polyureas obtainable in accordance with these prior publications are eminently suitable for the production of coatings. The effect of using blocked polyisocyanates instead of the free isocyanates is that the reaction mixtures have a processing time which is long enough for practical requirements. By contrast, corresponding reaction mixtures of the corresponding non-blocked polyisocyanates and the corresponding polyamines would be far too reactive to be used able as binders in two-component lacquers. However, in the case of the systems according to the above-mentioned prior art, the increase in processing time which is made possible by blocking of the isocyanate groups is offset by the disadvantage that the systems are too sluggish in reaction at room temperature to guarantee satisfactory hardening of the lacquer coatings produced therefrom.
Extremely effective catalysts for the above-mentioned reaction are described in German Offenlegungsschrift No. 2,722,514. However, the amidines proposed as reaction accelerators in this prior publication are attended by the disadvantage that they are sensitive to hydrolysis. The activity thereof in modified practical mixtures is lost in the event of prolonged storage so that they have to be added to the mixtures as a third component just before they are used.
Accordingly, an object of the present invention is to provide reaction accelerators which enable two-component coatings based on blocked polyisocyanates and polyamines to be obtained without the disadvantages attending the hydrolysis-prone amidines.
It has surprisingly been found that this object may be achieved by using compounds containing at least one guanidine group as accelerators for the above-mentioned reaction. In particular, it is possible by the process according to the present invention to produce storable, modified mixtures, for example, with coal tars, having hardening times at room temperature which satisfy practical requirements.